Red-emitting phosphors that are excited by light corresponding to near-UV to blue light-emitting diodes (LEDs) are needed to improve the color rendering properties of white LEDs or, when white LEDs are used as the backlights in liquid crystal displays, to enhance color reproducibility, and are being studied for this purpose. In particular, JP-A 2009-528429 (Patent Document 1) mentions that complex fluoride phosphors obtained by adding manganese to a complex fluoride of the formula A2MF6 (wherein A is, for example, sodium, potassium or rubidium, and M is, for example, silicon, germanium or titanium) are useful.
Patent Document 1 discloses one method for producing such phosphors wherein a hydrofluoric acid solution in which all of the constituent elements have been dissolved or dispersed is concentrated by evaporation, inducing precipitation. In another production method, disclosed in U.S. Pat. No. 3,576,756 (Patent Document 2), hydrofluoric acid solutions in which the respective constituent elements have been separately dissolved are mixed together, following which acetone (a water-soluble organic solvent) is added, lowering the solubility and thereby inducing precipitation. In addition, JP No. 4582259 (Patent Document 3) and JP-A No. 2012-224536 (Patent Document 4) disclose methods wherein the element M and the element A in the above formula are each dissolved in separate hydrofluoric acid-containing solutions, manganese is added to either one of these solutions, and the solutions are mixed together, thereby inducing precipitation of the phosphor.
Processes for producing the above known manganese-doped complex fluoride phosphors of the formula A2MF6 (wherein A is, for example, Na, K or Rb; and M is, for example, Si, Ge or Ti) mentioned above, including the processes described in the foregoing literature, use a considerable amount of high-concentration hydrofluoric acid relative to the amount of phosphor to be obtained in the phosphor-forming operation. Because hydrofluoric acid is strongly corrosive, it places limitations on, for example, the reactor materials, which may lead to problems when trying to carry out large-scale production. Moreover, hydrofluoric acid is strongly toxic to the human body and so, from the standpoint as well of concerns over the safety of workers handling this chemical, there are obstacles to scaling up chemical processes which use hydrofluoric acid.
In addition, it has been pointed out that the emission characteristics of such complex fluoride phosphors may deteriorate under high temperature and high humidity. Along with mentioning this, JP-A No. 2009-280763 (Patent Document 5) also states that the problem of moisture resistance can be alleviated by including a step that entails, at the time of LED fabrication, mixing and molding the phosphor together with a silicone resin. Also, JP-A 2010-45328 (Patent Document 6) teaches that the problem of moisture resistance is alleviated by covering these complex fluoride phosphors with a resin or the like prior to LED fabrication.
However, there exists a desire for an advantageous method to further increase the moisture resistance.